Extend query(True) and custom weights

[VincentDerk]

• When semiring.is_nsp(): query(True) now correctly computes WMC(Theory). For now, SDD and FSDD rely on ExplicitSDD for this.
• Custom weights support has been extended to allow both positive and negative weights e.g; {a: pn_weight(0.1,0.1)}.
• d-DNNF's weight dict has been changed to be the same as the SDD's.
• Semiring extended with to_evidence(..) and ad_negate(..) for more user flexibility. Default implementation is consistent with previous behavior.